The development of lightweight materials has gained increasing significance as car manufacturers strive to reduce vehicle weight in order to improve performance, lower fuel and oil consumption, and reduce emissions. To meet demands for weight reduction, and system consolidation in automotive vehicles, a one-piece door module was developed which integrates the window regulator, door motor, regulator channel, inside handle, speaker, door wire harness clip, etc., into a single unit. The manufacturing of a one-piece door module replaces conventional methods of door assembly, resulting in a much more streamlined process and improved productivity.
The technique mentioned above is evolving from part-by-part modulation for individual major parts to plate modulation, in which all parts are integrated into a steel plate. The present invention purports to replace the steel employed in the plate modulation with a lightweight plastic and provides a long-fiber reinforced polypropylene composition suitable for the purpose.
Polypropylene is widely used because of its superior chemical resistance and good moldability. However, it has poor thermal resistance and mechanical strength, and thus cannot be used for certain parts that require robust mechanical properties or thermal resistance. To overcome this problem, various kinds of organic or inorganic materials have been used to reinforce a polypropylene resin. For example, mechanical properties and thermal resistance of a polypropylene resin can be improved by mixing the polypropylene resin with glass fiber using such mixing apparatus as an extruder. However, the amount of improvement in rigidity is slight since most of the glass fiber is substantially cut up by the screw or other part of the mixing apparatus and reduced to short fragments of about 0.7 mm or smaller before being mixed in with the polypropylene resin.
There is increasing demand in the automotive and electronic industries for a polypropylene resin with good impact resistance and good rigidity. However, such need cannot be satisfied by the aforementioned short-fiber-reinforced polypropylene resin. To this end, several methods have been proposed in the art. For example, one method involves unraveling a fiber bundle from a fiber filaments cluster or a composite cluster of polypropylene and non-polypropylene fiber filaments followed by impregnating and/or coating the unraveled fiber filaments with polypropylene. Various other techniques are also known in the art. These include heating fiber filaments while adhering a powdery polypropylene suspension to the fiber filaments, impregnating polypropylene by contacting a melt resin to a fiber filaments, adhering polypropylene powder to a charged fiber filaments and heating it to impregnate the polypropylene, impregnating a polypropylene resin dissolved in a solvent to a fiber filaments and removing the solvent to obtain a polypropylene long-fiber reinforced resin in which each fine fiber filament is encoated with polypropylene. Long-fiber reinforced molding products produced from these matrix resins have been shown to demonstrate good mechanical strength and, in particular, impact strength.
WO 1996-006132, U.S. Pat. No. 5,985,971, Dave et al. (1996) Journal of Vinyl & Additive Technology vol. 2, No. 3 and Martinez Burgos et al. (2003) Journal of Polymer Science Part B: Polymer Physics 41(11): 1244 disclose compositions comprising fiber-reinforced polypropylene elastomers prepared by metallocene-catalyzed polymerization. These methods and compositions have however several drawbacks however; the mixing process by screw extrusion or kneading takes at least 30 sec and the resultant compositions exhibit very low rigidity albeit improved impact resistance. Moreover, the compositions of WO 1996-006132 and U.S. Pat. No. 5,985,971 have a very low glass fiber content (1-10 wt %) and the glass fibers that are present are not readily compatible with resin.
U.S. Pat. No. 6,747,094 discloses the use of a crosslinkable ethylene-octene or ethylene-butene rubber prepared using metallocene catalysts and having a diene moiety. Although this technique is effective in improving impact resistance, the rigidity and flow property of the resultant composition worsen considerably as the resin penetrates into the layer of long-fiber filaments within a short time, thereby making impregnation very difficult.
Korean Patent Publication No. 10-2001-0051300, Japanese Patent No. Hei 10-377454, Japanese Patent No. 2000-328717, Japanese Patent No. 2001-32050, U.S. Pat. Nos. 5,409,763 and 5,866,648 disclose methods of adding a non-olefinic resin with high rigidity to a polyolefin-based resin and/or forming an alloy. Most of the methods either form an alloy of polyamide with a polypropylene resin or improve physical properties of the polypropylene-based resin using polyethylene terephthalate, polybutylene terephthalate, polyoxymethylene, polyphenlyene sulfide, styrene/acrylonitrile copolymer, acrylobutadiene-styrene copolymer, polycarbonate, polymethyl methacrylate, etc. However, none of these compositions demonstrate a high level of resistance to damage from external impact or superior rigidity when subject to an external load as do the compositions of the present invention.